Soft X-ray observations by Yohkoh SXT have revealed that transient brightenings of small coronal loops frequently occur in active regions. Their estimated energy is $10^{24} \sim 10^{28}$ ergs and non-thermal behaviors may be observed in the large member of the brightenings. They are considered as soft X-ray signatures of microflares observed in hard X-rays. Lower coronal temperature plasma (1-2MK) produced by Yohkoh transient brightenings is simultaneously observed in EUV wavelengths by TRACE and SoHO EIT/CDS. Moreover, EUV observations have shown a lot of tinier transient brightenings without accompanying SXT transient brightenings. Coordinated observations with photospheric observations show that the location of Yohkoh transient brightenings is well localized in active regions, i.e., in emerging flux regions, around well-developed sunspots, and in the coronal bundles connecting the leading plage to the following plage regions. In some Yohkoh events, small-scale emergence of magnetic flux is well associated with their occurrence, suggesting that emerging flux plays a key role in triggering transient energy release in the corona. This paper reviews multi-wavelength observations coordinated with Yohkoh SXT observations for investigating the nature of SXT transient brightenings (SXR microflares) and their associated emerging flux. Finally, Solar-B space observatory is briefly introduced as a next powerful tool for multi-wavelength investigations.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html